There has been a great deal of work on the refinement of microstructures based on cold-working and recrystallizing heat treatments (annealing). Unfortunately, these techniques have experienced limited success for refining pure aluminum microstructures. The highly-mobile grain boundaries in high-purity aluminum can allow spontaneous partial recrystallization to occur at room temperature under normal, ambient working conditions. In addition, high-purity aluminum does not have any precipitates or any significant amount of solute to provide the xe2x80x9cZener dragxe2x80x9d necessary for effective retardation of grain boundary motion. Consequently, grain size is very difficult to control using conventional thermomechanical processing methods.
Historically, pure aluminum sputter targets have been manufactured with recrystallized grain sizes ranging typically from 500 xcexcm to 5 mm. These xe2x80x9clargexe2x80x9d grain sizes can contribute to poor sputter uniformity. In addition, since these pure aluminum sputter targets have limited strength, they often require backing plates to control warping during sputtering. In view of these problems, there is a desire to improve the strength and sputtering performance for high-purity aluminum targets.
Target manufacturers have relied upon equal channel angular extrusion (ECAE) to produce fine grain microstructures. Nakashima et al., xe2x80x9cInfluence of Channel Angle on the Development of Ultrafine Grains in Equal-Channel Angular Pressing,xe2x80x9d Acta. Mater., Vol. 46, (1998), pp. 1589-1599 and R. Z. Valiev et al., xe2x80x9cStructure and Mechanical Behavior of Ultrafine-Grained Metals and Alloys Subjected to Intense Plastic Deformation,xe2x80x9d Phys. Metal. Metallog., Vol. 85, (1998), pp. 367-377 provide examples of using ECAE to reduce grain size. ECAE introduces an enormous strain into a metal without imparting significant changes in workpiece shape. Although this process is effective for reducing grain size, it does not appear to align grains in a manner that facilitates uniform sputtering or provide an acceptable yieldxe2x80x94the low yield originates from the ECAE process operating only with rectangular shaped plate and thus, requiring an inefficient step of cutting circular targets from the rectangular plate.
Another mechanical method for producing fine grain structures in metals is xe2x80x9caccumulative roll bondingxe2x80x9d where aluminum sheets are repeatedly stacked and rolled to impart sufficient strain required for ultra-fine grain sizes. N. Tsuji et al., xe2x80x9cUltra-Fine Grained Bulk Steel Produced by Accumulative Roll Bonding (ARB) Process,xe2x80x9d Scripta. Mater., Vol. 40, (1999), pp. 795-800. The repeated stacking and rolling allows rolling to continue after the aluminum reaches a critical thickness. Although this process is useful for producing some products, it is not necessarily applicable for sputtering targets because of material purity requirements.
Researchers have explored using cryogenic working to increase the forming limits of aluminum alloy sheet panels. For example, Selines et al. disclose a cryogenic process for deforming aluminum sheet in U.S. Pat. No. 4,159,217. This cryogenic process increases elongation and formability at xe2x88x92196xc2x0 C. In addition, similar work has focussed on increasing the formability of sheet panels for automotive applications. Key references include: i) H. Asao et al., xe2x80x9cInvestigation of Cryogenic Working. I. Deformation Behaviour and Mechanism of Face-Centered Cubic Metals and Alloys at Cryogenic Temperature,xe2x80x9d J. Jpn. Soc. Technol. Plast., Vol. 26, (1985), pp. 1181-1187; and ii) H. Asao et al., xe2x80x9cInvestigation of Cryogenic Working. II. Effect of Temperature Exchange on Deformation Behavior of Face-Centered Cubic Metals and Alloys,xe2x80x9d J. Jpn. Soc. Technol. Plast., Vol. 29, (1988), pp. 1105-1111.
Lo, et al., in U.S. Pat. No. 5,766,380, entitled xe2x80x9cMethod for Fabricating Randomly Oriented Aluminum Alloy Sputtering Targets with Fine Grains and Fine Precipitatesxe2x80x9d disclose a cryogenic method for fabricating aluminum alloy sputter targets. This method uses cryogenic processing with a final annealing step to recrystallize the grains and control grain structure. Similarly, Y. Liu, in U.S. Pat. No. 5,993,621 uses cryogenic working and annealing to manipulate and enhance crystallographic texture of titanium sputter targets.
The invention is a high-purity aluminum sputter target. The sputter target is at least 99.999 weight percent aluminum and has a grain structure. The grain structure is at least 99 percent recrystallized and has a grain size of less than 200 xcexcm.
The method of the invention forms high-purity aluminum sputter targets by first cooling a high-purity target blank to a temperature of less than about xe2x88x9250xc2x0 C. The high-purity target blank has a purity of at least 99.999 percent and grains of a grain size. Then deforming the cooled high-purity target blank introduces intense strain into the high-purity target blank. And recrystallizing the grains at a temperature below about 200xc2x0 C. forms a target blank having recrystallized grains. The target blank has at least about 99 percent recrystallized grains; and the recrystallized grains have a fine grain size. Finally, finishing the high-purity target blank at a low temperature sufficient to maintain the fine grain size forms a finished sputter target.